feat(gallery): sort by earliest post date across all posts (new default)

The gallery's newest/oldest sort keys off image_record.effective_date =
COALESCE(primary post's post_date, created_at). The primary post is often the
repost/download the file came from, so the grid led with download dates rather
than when content was first posted (operator-flagged).

Add a second materialized sort key, earliest_post_date = MIN(post_date) across
ALL of an image's provenance posts (every post it appears in), else created_at —
the original publish date. Mirrors the effective_date pattern so the sort stays a
forward index scan.

- alembic 0071: add earliest_post_date + index (DESC, id DESC); backfill
  created_at baseline then MIN over image_provenance ⋈ post.
- importer: recompute earliest_post_date whenever a dated post is linked (MIN over
  the image's provenance, which now includes the just-added row).
- gallery_service: new sorts posted_new / posted_old key off earliest_post_date;
  cursor + year/month grouping follow the active column transparently.
- api: accept posted_new|posted_old; DEFAULT is now posted_new so the grid leads
  with original publish date. newest/oldest (effective_date) still available.
- frontend: sort dropdown gains "Newest/Oldest post date" (default Newest post
  date); existing effective-date sorts relabelled "Newest/Oldest added".
- tests: service test asserts posted_new/posted_old key off earliest_post_date;
  frontend default-sort omission test updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

frontend/test/gallery.spec.js

@@ -43,7 +43,7 @@ describe('gallery store: composable filter', () => {
     expect(scroll).toContain('sort=oldest')
   })
 
-  it('omits sort=newest and sends no filter params when empty', async () => {
+  it('omits the default sort and sends no filter params when empty', async () => {
     const s = useGalleryStore()
     const urls = []
     stubFetch((url) => { urls.push(url); return { status: 200, body: EMPTY } })
@@ -197,7 +197,7 @@ describe('gallery store: composable filter', () => {
 describe('filterToQuery / cloneFilter', () => {
   it('serializes faceted params incl. platform sentinel and flags', () => {
     const q = filterToQuery({
-      tag_ids: [3], artist_id: null, media_type: null, sort: 'newest',
+      tag_ids: [3], artist_id: null, media_type: null, sort: 'posted_new',
       platform: '__unsourced__', untagged: true, no_artist: false,
       date_from: '2024-01-01', date_to: null,
     })
@@ -207,7 +207,7 @@ describe('filterToQuery / cloneFilter', () => {
     expect(q.no_artist).toBeUndefined()
     expect(q.date_from).toBe('2024-01-01')
     expect(q.date_to).toBeUndefined()
-    expect(q.sort).toBeUndefined()  // newest is the default → omitted
+    expect(q.sort).toBeUndefined()  // posted_new is the default → omitted
   })
 
   it('serializes similar_to', () => {